Substances required for the manufacture of semiconductors are often toxic and/or chemically aggressive. In fiber optic technology, toxic and chemically aggressive acid halides are typically stored in glass containers or stainless steel vessels. Impurities in the containers which are used for the storage and transport of these substances can lead to considerable problems and must therefore be excluded. For such applications, chemically resistant spool valves can be used to withdraw the product using a protective gas, such as an inert gas, to displace the product from or carry it out of the container. In most cases the protective gas is also used for flushing the connected lines after withdrawal. Steel ball valves may be used for withdrawal of product from stainless steel vessels. For glass vessels, plug valves have been used. Although plug valves allow flushing with the protective gas, they may not seal adequately. Other glass and/or PTFE valves have also been used for applications of this type.
One particular valve is a spool valve as disclosed in U.S. Pat. No. 4,805,675, which is hereby incorporated by reference. The spool valve of the '675 patent has a cylindrical valve spool which is axially moveable in a bore through a valve housing. The valve spool has a first annular groove in its outer surface which, in the open position of the valve, connects a product supply line (e.g., a diptube) to a product withdrawal line (i.e., a line to a location where the product is to be delivered). The valve spool also has a second annular groove in its outer surface which, in the open position of the valve, connects a protective gas supply line (e.g., a line from a pressurized supply of protective gas to the valve) to a gas injection line (i.e., a line from the valve into the container). A third annular groove is formed in the outer surface of the valve spool so that the first annular groove is between the second and third annular grooves along the axis of the spool. The third annular groove is connected to the second annular groove by a bypass passageway through the valve spool so the third annular groove is filled with gas having a pressure about equal to the pressure in the first annular groove. Thus, the second annular groove, which is used for passage of the product through the valve, is flanked on both sides by the protective gas in the first and third annular grooves, thereby (according to the '675 patent) reducing the risk of contamination leaking into the second groove from outside the container and reducing the risk that product will leak from the second groove out of the valve through the joint between the spool and the housing. Further, when the valve is in its closed position, the second annular groove remains in fluid communication with the gas supply line, but is sealed from fluid communication with the gas withdrawal line. The third annular groove is also connected to the product withdrawal line when the valve is in its closed position, which results in flushing of the product withdrawal line with the protective gas to purge product from portions of the valve and the product withdrawal line.
The valve disclosed in the '675 patent has some disadvantages. First, it is expensive to manufacture the annular grooves in the valve spool. Also manufacture of the bypass passageway requires formation of a longitudinal bore, formation of two radial bores, and plugging of the end of the longitudinal bore. Further, it would be desirable to reduce the amount of internal space in the valve in which the product stagnates (i.e., dead space). Dead space is undesirable because the aggressive product can react with the materials used in the construction of the valve, in particular by leaching metals or other extractable contaminants from the valve components between uses of the valve. Dead space also increases the risk of cross contamination of different batches of product when the same valve is used to transfer product from containers holding the different product batches. Moreover, the ports for connection of the product supply tube and the gas withdrawal line are confusingly similar in appearance increasing the risk that the product supply tube and gas withdrawal line will not be properly connected to the valve for the intended application. Still further, when the valve is used with a toxic chemical product there is concern that the product might leak out of the valve through the joint between the valve spool and the valve housing.